Prenatal maturation of endocochlear potential and electrolyte composition of inner ear fluids in guinea pigs.

The maturation of the endocochlear potential (EP) and the inner ear fluid ionic composition were studied in fetal and neonate guinea pigs. The concentration of sodium and potassium in endolymph and perilymph approximated adult values more than 2 weeks before birth. Endolymph had acquired its specific ionic composition before the onset of cochlear microphonics (52-55 gestation days, results of other authors). Positive EP was recorded starting on day 62 of gestation. The EP rose fast to reach near adult level at birth. It is speculated that the negative EP recorded after the onset of cochlear microphonics was an artifact, probably a result of fetal hypoxia. The significance of the negative EP recorded before the onset of the cochlear microphonics is discussed in relation to the source of the anoxic negative EP in the adult animal.